Bull,
I think widespread metric tool availability coincided with the sale of Japanese and European (not British) cars and motorcycles in the U.S. market. This would have started in the early 1960's and really taken off in the 1970's.
If you look at the old Sears/Craftsman catalogs as a guideline for metric tool introduction into the consumer market, you can see the trend unfolding:
1959: metric tools first introduced in small, limited sets of 1/4" and 1/2" sockets, drive tools, and DOE and DBE wrenches.
1964: 3/8" metric socket sets were added to the previous offerings above.
1969: metric hex keys and combo wrenches were added to the lineup. Individual metric sockets and wrenches were added to open stock in addition to previous tool set offerings above.
Although metric tools were available much earlier in the automotive trade and industrial tool markets, I'm guessing that they were not sold in large quantities until the 1960's, paralleling what was going on in the consumer market.
This would explain the absence or scarcity of metric tools among vintage tools, say pre-1980.
Your thoughts?